1. Field of the Invention
The present invention relates to the detection of objects moving underground. It is particularly, but not exclusively, concerned with the case where the moving underground object is a boring tool.
2. Summary of the Prior Art
It is already known, from eg WO95/30913 for antennas in a boring tool to be used to detect the electromagnetic field generated from a buried current-carrying conductor. The use of multiple antennas enables the relative positions of the boring tool and conductor to be determined to prevent the boring tool contacting the conductor, which may damage the conductor. WO95/30913 also discloses that it is possible to control the drilling force of the boring tool in dependence on the separation of the boring tool and the conductor.
The arrangement described in WO95/30913 depends on the conductor carrying a current of sufficient magnitude and frequency that the magnetic field generated may be detected at a sufficiently large distance from the conductor. The technique is therefore not suitable for preventing contact between a boring tool and an object which does not carry current.
In WO 96/29615 corresponding to U.S. Ser. No. 08/894664, the disclosure of which is incorporated herein by reference, an arrangement was disclosed in which an underground boring tool had a solenoid on or in it, which solenoid generated a magnetic field which could then be detected remotely by suitable antennas. In such an arrangement, by analysing the signals at the antennas, the relative position of the antennas and the boring tool could be determined.
The present invention, at its most general, proposes that a monitoring device is provided which is able to detect a moving underground object, and the monitoring device defines at least one boundary which the moving underground object should not cross. The monitoring device may then be connected to the drive to the moving underground object to inhibit or alter movement of the underground object when that boundary is reached, or an alarm can be generated when the underground object reaches the boundary.
This enables the monitoring device to define a protection zone into which the underground object cannot. enter. That protection zone is defined by the monitoring device, and therefore the monitoring device needs to be positioned so that a buried object which the moving underground object should not contact is within the protection zone. However, assuming a suitable size of protection zone, exact positioning of the monitoring device relative to the buried object is not needed. Moreover, any type of buried object can be protected, not merely current carrying objects.
As mentioned above, the monitoring device must define at least a boundary, and preferably a protection zone, which the moving underground object should not cross. The monitoring device therefore needs to be able to define the boundary(s), and also needs to determine the position of the moving underground device in relation to the boundary(s). This can be done, for example, by determining the position of the moving underground object relative to the monitoring device, and comparing that position with the position(s) of the boundary(s).
Many different techniques can then be used to determine the position of the moving underground object. For example, if the monitoring device contains two antennas, the techniques disclosed in WO96/29615 can then be used. Since the techniques disclosed in WO96/29615 are based on the assumption that the measurements are carried out in a plane containing the axis of a solenoid in the moving underground object, those techniques are suitable for use when the boundary is transverse to that plane. In such circumstances, even if the monitoring device is not on the axis, the calculations made (which assume it is) will nevertheless determine the relative position of the underground moving object relative to the boundary.
The techniques disclosed in WO96/29615 depend on comparison of measured values of the axial radial component of the magnetic field from the solenoid with stored information relating to the relationship between the axial radial components. By comparison of the measured values with those stored, the relative position of the solenoid (and hence the moving underground object) and the monitoring device can be determined.
However, other techniques for determining the position of the solenoid in the underground moving object may be used. For example, and as discussed in GB 2330204 it is possible to detect the vertical and horizontal fields from a solenoid and to use those values to predict the ratio of the vertical and horizontal fields at a position vertically above (or below) the solenoid. In GB 2330304, corresponding to U.S. Ser. No. 09/168414, the disclosure of which is incorporated herein by reference, it was then assumed that the detector was moved until those two values coincide. Then, the detector was vertically above the underground moving object. However, in the present invention, the same techniques can be used by applying a correction based on the relative position of the monitoring device and the boundary. If the information derived by the monitoring device is used to consider the relative position of the boundary and the solenoid, so that coincidence occurs when the solenoid is vertically above (or below) the boundary, the effect of the present invention can be achieved.
In such an arrangement, the vertical and horizontal fields strengths are measured using a detector having at least one vertical, and at least one horizontal detecting antenna. From those measurements, the ratios of the field strengths is determined, thereby to determine the distance between the detector and the solenoid. Also obtained is the tilt of the solenoid, which should be derived from eg a tilt sensor mounted on the moving underground object. Using those measurements, a prediction of the ratio of the vertical and horizontal field strengths directly above or below the solenoid can be determined, and compared with a corrected measurement value, being the value of that would be measured at the boundary. Then, as the moving underground object moves towards the boundary, the predicted and measured values will eventually coincide.
In such an arrangement, there normally needs to be an established relationship between at least the horizontal antenna of the detector and the solenoid, so that the orientation of the fields of the solenoid and the horizontal coil antenna are the same. This enables the detector to be given the correct orientation relative to the solenoid, since otherwise the comparison of the predicted and measured values of the horizontal and vertical fields may not coincide at the right place, at least when the solenoid is tilted.
The direction of the movement of the underground moving object needs to be determined. There are several ways of doing this. For example, as a boring tool moves from an initial position to a final position, this defines a movement direction. However, it is not always practical to use the start position. Therefore, the monitoring device may measure the field strengths from the solenoid, detect movement of the solenoid, and compare the measured field strengths at successive locations. If the field strength is increasing, the solenoid is moving towards the monitoring devices. Another alternative however, is to make use of the measured ratio of vertical to horizontal field strengths and the predicted value of that ratio directly above the solenoid. The variation of those two values away from the position directly above the solenoid is predictable, and can be used to determine the direction towards the solenoid. This arrangement has the advantage that it does not involve comparison between measured values of the field strengths.
A further complication is that, for some tilt angles of solenoid, there may be a position which is not directly above the solenoid, but for which the measured and predicted values of the ratio of field strengths nevertheless coincide. If that position exists at all, it is relatively remote from the solenoid. Therefore, it is preferable that a detection region is defined proximate the point directly above the solenoid, and an arrangement is provided for detecting when the detector is within that detection region, or for permitting operation of the detector only within the detection region. Then, provided the boundary is only within that detection region, there is only one point at which the measured and predicted values of the ratio coincide.